This invention relates to the detection of access bursts in TDMA (Time Division Multiple Access) cellular radio or wireless communications systems.
In a cellular mobile radio communications system using TDMA, it is known to adjust the timing of time slot bursts transmitted from each mobile station to a base station so that bursts received by the base station from different mobile stations are aligned in time in their respective time slots. This time alignment typically comprises a fixed or standard offset time between reverse and forward time division multiplex frame timing (forward refers to the downlink transmission direction from the base station to the mobile station, and reverse refers to the opposite or uplink direction of transmission), and a time alignment advance (or retard) which varies according to the signal propagation time (i.e. the distance) between the mobile station and the base station.
In order to determine an initial time alignment for a mobile station newly communicating with a base station, the base station provides to the mobile station an initial traffic channel designation and instructs the mobile station to transmit access bursts, also referred to as shortened bursts, on this channel using the standard offset. The base station then attempts to monitor the access bursts and to determine from their timing an appropriate time alignment advance for the mobile station. This process is complicated by the fact that at this time the carrier offset, sample timing offset, and channel impairments are unknown at the base station receiver. The base station then communicates the time alignment advance to the mobile station as an initial time alignment for the mobile station to use for subsequent transmission of normal bursts to be received at the base station with the correct time relationship.
By way of example, in a TDMA cellular radio system that operates compatibly with EIA/TIA document IS-54-B: Cellular System Dual-Mode Mobile Stationxe2x80x94Base Station Compatibility Standard (Rev. B), referred to herein simply as an IS-54 system, the access (shortened) burst has a predetermined form, described further below, comprising guard and ramp times, a sync word S, a CDVCC (coded digital verification colour code) word D, and fields V, W, X, and Y that comprise different numbers of zero bits. The sync word S is one of six defined sequences of 14 symbols identifying respective time slots and also used for synchronization and equalizer training. The CDVCC word D comprises 6 symbols (12 bits) having any of 255 distinct values used to identify a cell. Consequently, the entire content of the access burst, which comprises repeated occurrences of the words S and D, is known to the base station. The access burst has different numbers of symbols between any two occurrences of the sync word, so that the initial time alignment for the mobile station can be determined by the base station after detection of any two or more sync words of the access burst.
Having instructed a newly communicating mobile station to transmit access bursts with a particular sync word, the base station expects to detect this sync word in such an access burst. If, during the period that the base station is monitoring received signals to detect the expected sync word, this same sync word is produced as part of a burst from a different mobile station in a neighbouring cell of the system, then the base station may falsely lock onto this. In consequence, it may determine an incorrect initial time alignment for the newly communicating mobile station and transmit this with a message to the newly communicating mobile station to instruct it to transmit normal bursts, resulting in undesired interference and the risk of disrupted communications. This problem is exacerbated with increasing numbers of mobile stations (for example at busy times of the cellular communications system) and with reducing cell sizes.
This problem can be avoided or eliminated by having the base station check more thoroughly the form of the burst which it receives and interprets as the access burst, for example by checking for more occurrences of the sync word and for the correct CDVCC word D. However, this requires more computation in the base station, and this presents a particular problem at the system""s most busy times when the computing resources of the base station may already be fully utilized.
A further problem can occur in that the accuracy with which the timing of the access burst, and hence the initial time alignment of the mobile station, is determined is adversely affected by multipath fading of the signal transmitted by the mobile station.
Accordingly, an improved method is desired for detecting access bursts, which avoids or eliminates the problems discussed above and does not increase, and advantageously decreases, the computing resources required of the base station.
An object of this invention is therefore to provide an improved method of and apparatus for detecting access bursts in a TDMA communications system.
According to one aspect, this invention provides a method of detecting an access burst in a TDMA communications system using differential modulation, the access burst including at least one predetermined sequence of NT consecutive identical symbols in a predetermined position in the burst, where NT is an integer greater than an integer L representing a number of symbol periods over which the system has channel dispersion, comprising sampling a received signal and detecting the predetermined sequence in the received signal by the steps of, for each of NT-L current symbol periods ending the predetermined sequence: predicting at least one sample of the received signal in the current symbol period from at least one sample of the received signal in the preceding symbol period assuming identity of symbols represented by the received signal in the current and preceding symbol periods; producing a squared distance between at least one sample of the received signal in the current symbol period and the predicted at least one sample; and comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.
In order to adjust the threshold for a fading channel such as in a wireless communications system, the method preferably includes the step of, for, each of the NT-L current symbol periods, producing the threshold in dependence upon a power of the received signal in the preceding symbol period.
The method preferably includes the step of accumulating the squared distance for a purality of the NT-L current symbol periods. The steps are desirably performed for each of a plurality of predetermined sequences in the access burst, and each of the predetermined sequences can for example represent a zero bit field in the access burst. Thus, in particular, at least one predetermined sequence can comprise at least one of zero bit fields W, X, and Y of an access burst of an IS-54 system, together with any zero bit symbols at the end of a CDVCC word preceding, and any zero bit symbols at the beginning of a sync word following, each of said fields W, X, and Y, and preferably said steps are carried out for each of a plurality of said fields W, X, and Y.
Another aspect of the invention provides a method of determining timing of an access burst received in a TDMA wireless communications system using differential modulation, the access burst including at least one predetermined sequence of NT consecutive identical symbols in a predetermined position in the burst, where NT is an integer greater than an integer L representing a number of symbol periods over which the system has channel dispersion, comprising the steps of: sampling a received signal; for each of a plurality of possible timings of the access burst, detecting whether the predetermined sequence is present in the respective position in the access burst by performing the following steps (a) to (d) either for NT-L current symbol periods which end the predetermined sequence if the access burst has the respective timing or until the squared distance in step (d) for the current symbol exceeds the respective threshold: (a) predicting at least one sample of the received signal in the current symbol period from at least one sample of the received signal in the preceding symbol period assuming identity of symbols represented by the received signal in the current and preceding symbol periods; (b) producing a squared distance between predicted and received samples for the current symbol period; (c) producing a threshold in dependence upon a power of the received signal for the preceding symbol period; and (d) comparing the squared distance with the threshold; and determining the timing of the access burst in dependence upon results of the detection of the predetermined sequence for said possible timings of the access burst.
The step of determining the timing of the access burst in dependence upon results of the detection of the predetermined sequence for said possible timings of the access burst preferably comprises, for each of said possible timings for which steps (a) to (d) are carried out for NT-L current symbol periods without the squared distance in step (d) exceeding the respective threshold, accumulating the squared distance produced in each step (b), and determining as the timing of the access burst one of said possible timings having a least accumulated squared distance.
Advantageously, said steps of detecting whether the predetermined sequence is present in the respective position in the access burst are performed, for each of the plurality of possible timings of the access burst, for each of a plurality of predetermined sequences in respective predetermined positions in the access burst. For example the at least one predetermined sequence can comprise at least one of zero bit fields W, X, and Y of an access burst of an IS-54 system, together with any zero bit symbols at the end of a CDVCC word preceding, and any zero bit symbols at the beginning of a sync word following, each of said fields W, X, and Y.
The invention also provides a method of detecting an access burst in a TDMA communications system not using differential modulation, the access burst including at least one predetermined sequence of NT consecutive identical symbols in a predetermined position in the burst, where NT is an integer greater than L+1 and L is an integer representing a number of symbol periods over which the system has channel dispersion, comprising sampling a received signal and detecting the predetermined sequence in the received signal by the steps of, for each of NT-(L+1) current symbol periods ending the predetermined sequence, producing a squared distance between samples of the received signal in the current and preceding symbol periods, and comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.
Again in this case, the method can include the steps of, for each of the NT-(L+1) current symbol periods, producing the threshold in dependence upon a power of the received signal in the preceding symbol period, and accumulating the squared distance for a plurality of current symbol periods, and the steps can be performed for each of a plurality of predetermined sequences in the access burst.
Viewed in a manner that is generic regardless of whether or not differential modulation is used, the invention provides a method of detecting an access burst in a TDMA communications system, the access burst including a predetermined sequence of consecutive identical symbols in a predetermined position in the burst, comprising detecting the predetermined sequence by the steps of, for each of a plurality of current symbol periods of a subset of the consecutive identical symbols ending said predetermined sequence and for which a received signal waveform can be predicted from a received signal waveform in a preceding symbol period: producing a prediction of the received signal waveform for the current symbol period from the received signal waveform for the preceding symbol period assuming identity of symbols in the current and preceding symbol periods; producing a squared distance between the prediction and the received signal waveform for the current symbol period; and comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.
The number of samples that can be predicted from respective preceding samples, and hence the size of the subset of consecutive identical samples ending the predetermined sequence which can be detected, is dependent upon the modulation scheme used and the channel dispersion, as well as the size of the predetermined sequence.
The invention also provides apparatus for detecting an access burst received in a TDMA communications system, comprising: a buffer for samples of a received signal; means for producing a squared distance between samples of the received signal in a current symbol period and a preceding symbol period; means for producing a threshold dependent upon a power of the received signal in the preceding symbol period; means for comparing the squared distance with the threshold thereby to determine identity of symbols in the current and preceding symbol periods when the squared distance is less than the threshold; and control means for controlling selection of samples supplied from the buffer as the samples of the received signal in the current and preceding symbol periods, the control means being responsive to an output of the means for comparing for identifying samples of the received signal representing a predetermined sequence of consecutive identical symbols in the access burst.
When the TDMA communications system uses differential modulation, the means for producing the squared distance preferably includes means for predicting at least one sample for the current symbol period from at least one sample for the preceding symbol period in accordance with the differential modulation assuming identity of symbols in the current and preceding symbol periods.
The apparatus preferably includes means for accumulating the squared distance for a plurality of consecutive current symbol periods, and preferably the control means is arranged for controlling said selection of samples supplied from the buffer for identifying samples of the received signal representing each of a plurality of predetermined sequences of consecutive identical symbols in the access burst.